Vehicle wheels are, in general, made up of a metal cylindrical rim having, at its axial ends, annular flanges which define a channel for interlocking a tire. In particular, the side portions of the tire, so-called “beads”, are fitted snugly on the annular flanges.
Tire wear, or the presence of manufacturing faults, results in geometric irregularities of the rim and of the tire which, during use, are manifested as vehicle vibrations.
The need to perform periodic wheel balancing operations suitable for re-establishing optimal vehicle attitude, and overcoming such vibrations, is thus well-known.
Traditional balancing operations require the fitting of weights, generally made of lead, on predetermined points of the wheel and along the rim.
Such operations are usually done using balancing machines comprising a bearing structure supporting wheel grip and rotation means, of the type of a horizontal spindle that can be axially turned by means of motor means and onto which the wheel rim is keyed. Known balancing machines are disclosed in U.S. Patent Application Publication 2007/0069571 A1, published Mar. 29, 2007, and U.S. Patent Application Publication 2008/0053223A; published on Mar. 6, 2008.
The measurement of the wheel unbalance is read during rotation by specific electronic or electro-mechanical devices, such as force transducers, fitted along the horizontal spindle. Normally, each of the measurements is related to a respective angular position of the wheel on the rotation axis.
The fitting of the weights offsets only the effects of the irregularity of the wheel in the distribution of the centrifugal forces during the rotation but, does not, however, solve the problems tied to the geometry defects of the wheel itself.
For this reason, in modern balancing machines, the balancing operation is often preceded by a coupling optimisation procedure between the tire and the rim, in order to reduce the effects of the geometric irregularity of the wheels.
Such optimisation procedure generally contemplates a preliminary measurement phase of the radial deviation of the rim and the tire eccentricity during one or more complete wheel rotations, normally performed by means of suitable measuring sensors with or without contact (e.g., feelers or optical sensors).
After the acquisition of the eccentricity data comes the analysis of the acquired curves and the determination of the eccentricity vectors relating to the rim and the tire.
Such eccentricity vectors are generally calculated starting with the maximum or minimum radial deviation peak values or, alternatively, starting with the first harmonic values of the acquired curves determined according to known mathematical periodical function analysis methods.
The tire is then turned, with respect to the rim, until it reaches an optimal angular position, in which the predetermined eccentricity vectors of the rim and the tire are in contrast with one another so as to minimize the resulting wheel eccentricity vector. These optimisation procedures relating to the coupling between rim and tire nevertheless have a number of drawbacks.
In particular, the use of the eccentricity vectors, determined starting from the peak deviation values or from the first harmonic values of the acquired curves, limits the analysis to a single rim and tire eccentricity component and does not consider the total geometric eccentricities of the wheel.